This invention relates to a lead frame assembly having two welded frames for use in a semiconductor device of the lead on chip (LOC) structure in which the leads extend over the semiconductor chip.
A conventional lead frame assembly for a semiconductor device of the LOC structure comprises a die pad frame having a die pad and a lead frame having leads resistance welded together as disclosed in Japanese Patent Laid-Open No. 5-343445. In FIG. 10a, which illustrates one example of this structure, reference numeral 1 is a lead frame, 2 are leads, 3 is a semiconductor chip, 4 is a bonding agent such as solder, 5a and 5b are a pair of electrodes, and 10 is a die pad frame. Resistance welding is a well-known method in which the welding regions are held under pressure between two electrodes 5a and 5b and an electric current is supplied therethrough to melt the welding regions of the frames 1 and 10 by Joule heating to weld them together. This welding method is advantageous in that two frames can be joined together quickly with a simple apparatus.
Japanese Patent Laid-Open No. 5-190720 discloses a method for bonding two frames into a frame assembly through the use of an energy beam such as a laser beam. In this method, a large pressure is not necessary, so that the deformation of the frame assembly due to the pressure is eliminated.
Since the frame 1 and/or 10 held together under pressure are melted by an electrical current in resistance welding, several problems occur. The frame assembly during welding is illustrated in FIG. 10b, in which the reference numeral 6 designates molten debris scattering from the molten portion of the frame and 1a and 10b designate bonding regions.
First, the frame 1 or 10 may be deformed or distorted during the welding process. That is, if the lead frame 1 and the die pad frame 10 are displaced relative to each other during the welding by several .mu.m parallel to the frames, the frame assembly is deformed into a convex or concave shape on the order of 100 .mu.m. In particular, after the entire welding steps are completed, the frame assembly as a whole has a large deformation due to the accumulated deformations and distortions. When such deformations are generated in the frame assembly, the next wire bonding process cannot be properly achieved due to the lead dislocation and insufficient heating of the die pad, resulting in a poor yield in the wire bonding process.
Second, the molten debris 6 scatters due to the abrupt change in temperature during welding. The molten debris 6 moves between the frames at a high speed and scatters outwardly in a molten state. When this high temperature molten debris 6 deposits on the semiconductor chip 3, the organic coating on the semiconductor chip 3 is melted and the circuit is short circuited, decreasing the yield of the manufactured semiconductor device. Also, if this molten debris 6 attaches to the electrode on the semiconductor chip 3, although this is not frequent, the wire bonding process cannot be satisfactorily performed.
Third, as illustrated in FIG. 11 which shows a flow of a shunted electrical current 7 during welding, a welding region 10b, first, second, third and fourth welding regions 10b1, 10b2, 10b3, and 10b4, respectively. When the first welding region 10b1 is to be welded, since the lead frame 1 and the die pad frame 10 are not yet electrically connected, an electrical current which is on the order of 500 A flows between the electrodes 5a and 5b shown in FIG. 10 and the shunted current is equal to or less than 1 A. However, when the second welding region 10b2 is to be bonded, since the frames to be welded are electrically connected together by the first welding region 10b1, an electrical current easily flows through the frame and, as shown in FIG. 11 at the arrow 7, one portion of the current flowing through the frame during the welding is shunted into the die pad (not shown) through the die pad support pins 10f1, 10f1' and also flows into the lead frame 1 through the die pad support pins 10f2 and 10f2' and through the first welding region 10b1. This current is of the order of 10 A, which may be sufficiently large to destroy the semiconductor chip.
On the other hand, with the laser welding method, while the problem of the chip destruction by the shunted current is solved since no electrical current is used as the energy source, other problems cannot be solved and unfortunately a larger amount of the scattering molten debris is generated in laser welding.
Accordingly, an object of the present invention is to provide a lead frame assembly in which a lead frame and a die pad frame are welded together and which is free from the above-mentioned problems of the conventional lead frame assembly.
Another object of the present invention is to provide a welded lead frame assembly in which deformation of the frame assembly is eliminated.
Another object of the present invention is to provide a welded lead frame assembly in which deformation of the frame assembly is eliminated and scattering of the molten weld debris onto the semiconductor chip can be eliminated.
Still another object of the present invention is to provide a welded lead frame assembly in which deformation of the frame assembly is eliminated and the scattering of the molten weld debris onto the semiconductor chip can be eliminated and in which the flow of electrical current into the die pad can be eliminated.
A further object of the present invention is to provide a welded lead frame assembly which is high in manufacturing yield and reliable.
With the above objects in view, the present invention resides in a lead frame assembly for a semiconductor device which comprises a lead frame and a die pad frame welded together. The lead frame includes a lead frame main body, at least one lead extending from the main body and a first welding region disposed to the lead frame. The die pad frame includes a die pad frame main body, at least one die pad supported from the die pad frame main body on which a semiconductor element is to be mounted and a second welding region disposed to the die pad frame main body and weld-bonded to the second welding region of the lead frame main body. At least one of the first and second welding regions comprise a welding pad welded to the other welding region and a support bridge connected between the welding pad and the frame main body for supporting the welding pad from the frame main body. The support bridge includes suppression means for suppressing transmission of at least one of mechanical force, heat and an electric current therethrough between the welding pad and the frame main body.
The lead frame assembly may further comprise a slit provided in the frame main body around the welding pad to substantially surround the welding pad, and the frame main body may have an inward facing edge in a spaced opposite relationship with respect to the welding pad. The support bridge may be connected between the welding pad and the inward side edge of the frame main body for supporting the welding pad from the frame main body.
The suppression means of the support bridge is a narrow width dimension of the support bridge which provides a resistance to the transmission of at least one of mechanical force, heat and electric current therethrough. The support bridge may have a bend for absorbing distortion of the welding region. The welding region may comprise a frame member having the inward facing edge which defines an outer boundary of the slit surrounding the welding pad.
The support bridge may be located, relative to the welding pad, on a side remote from the die pad to which the semiconductor element is to be mounted. The lead frame main body may comprise a lead frame member supporting the leads and disposed between the welding regions and have a rigidity greater than that of a frame member of the die pad frame corresponding to the lead frame member. The lead frame member of the die pad frame corresponding to the lead frame member may be provided therein with an expansible and contractible expansion section for absorbing the distortion.
The lead frame main body may comprise a lead frame member supporting the leads and the support bridge is separate from the lead frame member having the leads. The assembly may also comprise a barrier member disposed so that a plane containing at least a welded interface around the welding regions and a surface of the semiconductor chip is crossed. The barrier member may comprise a bent portion of the welding region frame located in the direction of scattering of the molten debris. The bent portion of the welding region frame defines a clearance between the lead frame and the die pad frame, and the barrier is positioned at an exit end of the clearance in the direction of movement of the molten debris.
The welding region may be located at about the midpoint between the opposing die pad support pins and, and the welding region may be located at about the midpoint between the opposing die pad support pins, and the impedances between the welding region and the pair of die pad support pins located at the opposite side with the die pad interposed therebetween may be made substantially equal to each other, so that the electrical potentials at each die pad support pins are equal to each other.